ESD Tutorial Includes:
ESD Basics How ESD affects electronics components Protecting against ESD ESD protected area ESD workbench ESD work mat ESD wrist strap ESD clothing ESD storage ESD process ESD on a budget Design to combat ESD
Electrostatic Discharge or ESD is a fact of everyday life and it is of particular importance in the electronics industry these days. Years ago when valves were used it was not a problem, and even with the introduction of transistors few considered it a problem. However when MOSFETs were introduced their failure rates rose, the problem was investigated and it was found that static build up was sufficient to cause the oxide layer in the device to fail. Since then the awareness of ESD has risen considerably because it has been shown to have an effect on many devices. In fact many manufacturers today consider all components to be static sensitive, not just MOS devices that are the most prone to damage. As a result of the importance attached to ESD manufacturers of electronics equipment spend many thousands of pounds to ensure their workplaces are protected against the effects of static. They ensure that the products they manufacture do not have high failure rates during manufacturing test, and are able to demonstrate a high reliability over a long period of time.
What is ESD?
Static is simply the build up of charge between two surfaces. It arises when surfaces rub together and this results in an excess of electrons on one surface and a deficiency on the other. The surfaces on which the charge builds up may be considered as a capacitor. The charge will remain in place unless it has a path through which it can flow. As there is often no real path through which the charge can flow the resultant voltage may remain in place for some time and this gives rise to the term "static electricity". However when a conduction path does exist a current will flow and the charge will be reduced. There is a time constant associated with the discharge. A high resistance will mean that a smaller current will flow for a longer time. A low resistance will give rise to a much faster discharge.
Obviously the levels of voltage and current which are produced depend of a large variety of factors. The size of the person, the level of activity, the object against which the discharge is made, and of course the humidity of the air. These all have a pronounced effect so it is almost impossible to predict the exact size of the discharges that will occur. However one of the major factors that affects the voltages that are produced is the types of material that are being rubbed together. It is found that different materials give different voltages. The voltage produced is dependent upon the position of the two materials in a series known as the tribo-electric series. The further apart they are in the series, the greater the voltage. The one that is higher up the series will receive a positive charge, and the one lower down a negative charge. Looking at the tribo-electric series list below it can see that combing ones hair with a plastic comb will give rise to a positive charge on the hair, and the comb will become negatively charged.
There are many ways in which charges can be built up. Even walking across a carpet can give rise to some very large voltages. Typically this might give rise to potentials of 10 kV. In bad cases it could even lead to potentials of three times this value. Even the act of walking across a vinyl floor may lead to potentials of around 5 kV being generated. In fact any form of movement where surfaces are rubbing together will lead to the generation of static electricity. Someone working at a bench using electronic components could easily generate static potentials of 500 V or more.
While most of these voltages seem to be very high, most of them pass un-noticed. The smallest discharges that can be felt are around 5 kV, and even these can only be felt on occasions because the current associated with them is very small. If the voltages rise to values around 10 kV the discharges can sometimes be seen in the dark. However when values rise above 20 kV they start to be felt more acutely, especially when there is a significant amount of charge stored. The discharges may only take a short time, often only a few picoseconds. However for the charge to be dissipated in this time the levels of current can reach several tens of amps explaining why some discharges feel distinctly uncomfortable.
There are several ways in which static charges can be transferred to semiconductor devices resulting in damage from ESD. The most obvious is when they are touched by an item that is charged and conductive. The most obvious example of this possibly occurs when a semiconductor is on a work bench and someone walks across the floor building up a charge and then picks it up. The charged finger then imparts the static charge very quickly to the semiconductor with the possibility of damage. Tools can possibly be even more harmful. Metal screwdrivers are even more conductive and will impart the charge even faster and this results in higher levels of peak current.However it is not necessary to touch components to cause damage to them. Items such as plastic cups carry a very high charge, and placing one of these near an IC can "induce" an opposite charge into the IC. This too can damage the semiconductor device. Ties made of man-made fibre are also an ESD hazard because they can charge up and easily hang near sensitive electronic equipment.